While the use of heat exchangers to cool lubricating oil employed in an internal combustion engine has long been known, the invention of the so-called "donut" oil cooler by Donald J. Frost as exemplified in his U.S. Letters Pat. No. 3,743,011 issued July 3, 1973 began a whole new era of vehicular oil coolers. Through Frost's invention, for the first time, it was possible to readily adapt a lubricating oil system of an internal combustion engine to include an oil cooler. Donut oil coolers of the Frost type have an axial length of only a couple of inches or less and are constructed so that, with the assistance of an adapter or pipe, they may be interposed between the engine block and the oil filter, being attached directly to the block in the location formerly occupied by the oil filter. All else that need be done is to connect to coolant ports on the housing of the donut oil cooler into the vehicular cooling system which is simply accomplished with hoses.
Donut oil coolers of this type typically include a housing which is connected to receive coolant and which contains a stack of relatively thin, disc-like chambers through which the oil to be cooled is circulated. In terms of plumbing, such oil coolers may be located upstream of the filter, in which case they are cooling dirty oil, or downstream of the filter, in which case they are cooling clean oil. Because such donut oil coolers typically include turbulators within the chambers through which the oil is circulated, it is most advantageous that they be located downstream of the filter to cool clean oil so that there is a lesser tendency of the turbulators to be gummed up by dirty oil to impede the flow of oil, and thus heat transfer, on the oil side of the oil cooler.
In the above-identified patent to Frost, there is disclosed a means whereby oil to be cooled from the engine may be passed through closed passages within the oil cooler directly to the filter for filtering therein prior to being admitted to the oil receiving chambers that are in heat exchange relation with the engine coolant. As disclosed in the Frost patent, these passages are located radially outwardly of the center of the oil cooler, but somewhat radially inwardly of the periphery of the disc-like chambers. As a consequence, there exists a small volume between the peripheries of the chambers and the closed passageways for the oil enroute to the filter which are subject to stagnation. As is well known, turbulence plays a significant part in the rate of heat transfer between fluids. Thus, where areas of stagnant fluid exist, heat transfer is considerably reduced from what would occur if more turbulent flow was present.
The present invention is directed to providing a donut oil cooler of the type wherein the oil is flowed first through the oil filter so that the cooling of the oil is performed on cleaned oil and wherein areas of stagnation are avoided to maximize heat transfer efficiency.